The citraconylation of canine fibrinogen produced a soluble modified fibrinogen. The action of thrombin on this fibrinogen resulted in the formation of a soluble modified fibrin instead of the usual thrombin-induced fibrin clot. The electron microscopic examination of this citraconylated fibrin demonstrated the presence of mostly protofibrils and no thick or long fibers. This is indicative of a lack of branch points any time after the protofibril stage and also suggests that the protofibril stage may be an intermediate in the mechanism of fibrin assembly to a network of fibrin fibers. It appears that the added negative charges generated by the citraconylation process disrupts the overlapping formation of fibrin polymerization. The interaction of thrombin with platelets alters the surface of the platelet from a disc to a sphere, at which point aggregation occurs with the release of ATP. The rate of aggregation appears to be independent of thrombin concentration. The addition of hirudin 10 seconds after thrombin addition had no effect on the system; however, blocking thrombin initially prevents the activation of the platelets. The reaction, therefore, occurs by a 2-step mechanism, and only the fast first step is thrombin-dependent.